The USA cannot practice imperialism because it upsets the United Nations. If the USA starts imperialism, the UN can get mad, and we might risk upsetting Europe. Also, we can risk nuclear war. In the Middle East, there are a bunch of religious fanatics with weapons. If they get access to a nuclear weapon, and find out that that the USA is coming. We might risk nuclear war. If humanity is at risk, than we should not do it. This upsets humanity, and upsets the US.
I'm sorry I forfeited, something big in my life came up. I also was referring to the impact of the imperialism that happened, not if it was executed today. Long term effects- there were many, good and bad. Bad, cultures were damaged, natives were somewhat oppressed, and it caused many wars. Good, however; the overall well being, life expectancy, infant mortality, sickness levels, etc. were curbed quite a bit; schools, hospitals, and other public buildings were made; it opened many trade routes; education and technology improved majorly; tribal violence and supply shortages were stopped; combine all that, the countries were brought up to speed somewhat with the rest of the world. They may have not liked it, but looking now, they are better off.